The present invention relates to a method of making treads for the recapping of tires. The invention also relates to treads which are produced in accordance with the improved method.
It is well known that the internal surface of a tread must be roughened prior to bonding to the carcass of a used tire, either directly or with the interposition of a conventional intermediate layer or insert which is placed between the internal surface of the tread and the peripheral surface of the carcass. As a rule, the internal surface of a prefabricated tread is roughened shortly prior to bonding to a carcass or to an intermediate layer in order to avoid oxidation of the material of the tread along the uneven internal surface. The internal surface of the intermediate layer can be subjected to an analogous treatment prior to placing it against the peripheral surface of the carcass.
When a prefabricated tread is to be bonded to the carcass of a used tire, its internal surface is roughened for two reasons, i.e., to remove the oxidized layer of material along the internal surface and to enhance the bonding action during vulcanizing. This insures that raw rubber which is exposed along the roughened internal surface adheres strongly to the peripheral surface of the carcass when the recapping operation is completed. If the tread consists of widely different materals, or if the materials of the tread and carcass are different, the internal surface of the tread is normally subjected to a very pronounced roughening action so as to greatly increase the overall area of the internal surface by imparting to such internal surface a pronouncedly coarse finish which contributes to a superior bonding action and reduces the likelihood of cracking in the regions of transition between different material types.
It was further proposed to roughen the internal surface of a prefabricated tread immediately after the making. The freshly roughened internal surface is then coated with a layer of adhesive and is overlapped by a suitable foil which seals the roughened internal surface from the surrounding atmosphere. However, the roughening of the internal surface still represents a separate step which must be carried out upon completion of the prefabricated tread and contributes significantly to the manufacturing cost. Moreover, the roughening of successive treads is not reproducible with a requisite degree of accuracy, especially if the roughening action is to produce relatively narrow recesses and small protuberances which cannot be machined at an acceptable cost. Thin treads cannot be properly roughened in presently available machinery; therefore, the roughening is often performed by hand, especially in regions which cannot be readily reached by machine tools.
It was further proposed to vulcanize a tissue onto the internal surface of a prefabricated tread, especially a tread for large (tactical, combat and analogous) tires. The tissue is forcibly removed prior to bonding of the tread to the carcass of a tire. A drawback of such proposal is that the tissue can be applied only to relatively large and thick treads because the removal of tissue necessitates the exertion of a very large force which must be furnished by a suitable machine. Furthermore, the depth of recesses and the height of protuberances on the internal surface of the tread subsequent to removal of the tissue is relatively small so that the bonding action between the tread and the carcass is not satisfactory.